1. Field of the Invention
This invention relates to a process for preparation of alkylated p-benzoquinones from the corresponding alkylated phenols, in particular, trimethyl-p-benzoquinone (II) from trimethylphenol (I), by catalytic oxidation using as the catalyst a salcomine prepared by an improved process. ##STR1##
2. Description of the Prior Art
Trimethyl-p-benzoquinone (II) is a very interesting compound from a commerical standpoint, since trimethylhydroquinone, an important vitamin E precursor, can be prepared from it by means of reduction using a simple method. On a commercial level, (II) is obtained through the oxidation of trimethylphenol (I). Salcomine, a bis-salicylidene-alkylenediamino-cobaltous ion complex having the formula ##STR2## is a good catalyst for this oxidation (see DE-PS No. 17 68 063).
Numerous methods are known for the preparation of salcomine. A list of the known methods is to be found in Bull Soc. Chim. France 1976, Nos. 11-12, pp. 1717-21. In the known processes, the Schiff base derived from two moles of salicylaldehyde and one mole of ethylenediamine is reacted with a cobaltous salt either in an aqueous medium or in an aqueous organic medium. The presence of a buffer (for example, an aqueous acetic acid-sodium acetate solution or an alkanol-pyridine mixture) is recommended to better dissolve the Schiff base. The precipitating salcomine must then be filtered off, washed repeatedly, and dried. The disadvantage of this process is that the filtering, washing, and drying is very expensive on a commercial level and the resultant salcomine is not of uniform quality relative to its oxidation catalysis.
D. Aymes et al (Loc. Cit.) therefore suggests an improvement in the method of salcomine preparation wherein the salcomine is prepared in benzene, chloroform, or methylene chloride, using a cobalt compound soluble in these solutions--cobalt acetylacetonate. The disadvantages of this process are that it uses particularly expensive cobalt compounds, relies on physiologically hazardous solvents, and gives relatively poor yields of salcomine.
The invention herein is an improved process for the preparation of salcomine in which the disadvantages experienced in the prior art are no longer encountered.